1. Field of the Invention
The present invention concerns a sensor membrane of an optode, a method for the determination of gases in gas mixtures, and an apparatus for the determination of gases in gas mixtures.
2. Description of Related Art
The increasing demand for direct, reliable, and rapid determination of gas concentrations in gas mixtures, for example in medicine, environmental engineering, smoke and fire detection, the monitoring of chemical processes, or also for measuring the exhaust gases of internal combustion engines, is leading to greater interest in the development of novel gas sensors. Only at great cost and in relatively unreliable fashion, for example, has it hitherto been possible to use electrochemical sensors based on mixed oxide materials for controlling climate control systems in automobiles (A. Zeppenfeld et al., Sensor 97 Conference Report 1, p. 113 ff.). Also known, for example, is the measurement of oxygen partial pressure in the exhaust gases of internal combustion engines by means of "lambda probes," which are also based on ceramic materials. An overview of methods for determining gases using solid electrolytes is provided by the article by I. Lundstrom, Sensor and Actuators B 1996, 35-36, p. 11 to p. 19, which also, in particular, discusses production, miniaturization, and cross-sensitivity problems. An attempt to eliminate, in particular, cross-sensitivities in gas determination is described in the article by E. Hermanns in Sensors and Actuators 1984, 5, pp. 181 to 186. In this context, gases are determined via the change in the electrical conductivity of polymers caused by gas absorption. The change in the layer thickness of polymer films which can absorb gases is measured by means of interference reflection (EP 0 737 768 A2). U.S. Pat. No. 5,030,420 discloses an apparatus for determining oxygen in gaseous mixtures by means of an optical sensor, also referred to as an optode, in which the quenching luminescence of ruthenium(II) complexes is measured. Another route to the determination, in particular the optical measurement, of chemical compounds and ions is offered by "ionophores." These are lipophilic ligands which have the ability selectively to complex specific ions and to transport them through membranes by means of a carrier mechanism (EP 0 578 630 B1), so that various ions are selectively determined in solution and can be detected calorimetrically. Among these ionophores, the electroneutral ionically active substances have gained particularly wide applicability as components in ion-selective liquid membrane electrodes, for example as polymer liquid membrane electrodes in clinical analysis, as microelectrodes in electrophysiology, and in high-selectivity ion transport through artificial membranes. Recently, they have also been used in optode membranes for reversible optical detection of charged and electrically neutral species in solution (U. E. Spichiger et al., Proceedings SPIE--Int. Soc. Opt. Eng. 1991, 1510, 118). Pronounced selectivit-es were obtained, in particular, for the direct determination of H.sup.+, Li.sup.+, Na.sup.+, K.sup.+, Cs.sup.+, Mg.sup.2+, Ca.sup.2+, Sr.sup.2+, Ba.sup.2+, Tl.sup.+, UO.sub.2.sup.2+, Cl.sup.-, and CO.sub.3.sup.2-. Two possible principles were used in this context: on the one hand, only one ion-selective chromophoric or chromogenic ligand is used, which in contact with the species to be detected changes, in particular, its color. On the other hand, the combination of electroneutral ion carriers with "chromoionophores" in very thin polymer liquid membranes allows a determination of the optical activity of those cations and anions. Both principles are usable only in pH-buffered solutions. With none of these proposed solutions, however, was it possible to accomplish a simple and rapid accurate determination of gaseous species in gas mixtures, reliably and with no great need for complex equipment, while also avoiding cross-sensitivities.